1. Field of the Invention
The present invention relates to a lighting device used, for example, in a backlight of a liquid-crystal display apparatus, a surface light source including the lighting device, and a liquid-crystal display apparatus including the surface light source.
2. Description of Related Art
In a backlight of a conventional large-sized liquid-crystal display apparatus, a number of cold cathode tubes are disposed right under a liquid crystal panel, and these cold cathode tubes are used together with members such as a diffusing plate and a reflecting plate. In recent years, light emitting diodes (hereinafter referred to as “LEDs”) have been used as light sources for backlights. LEDs have increased their efficiency recently, and are expected to serve as low-power light sources to replace fluorescent lamps. In the case where LEDs are used as a light source in a liquid-crystal display apparatus, the power consumption of the apparatus can be reduced by controlling the light and dark states of the LEDs according to an image to be displayed.
In a backlight of a liquid-crystal display apparatus using LEDs as a light source, a large number of LEDs are disposed therein instead of cold cathode tubes. The use of a large number of LEDs allows the entire surface of the backlight to have uniform brightness, but the need for such a large number of LEDs is an obstacle to cost reduction. Attempts to increase the output power of each LED to reduce the required number of LEDs have been made. For example, JP 3875247 B proposes a surface light source in which a lighting device including a lens for widening a range of transmission directions of LEDs is disposed behind a diffusing plate.
For another example, JP 2002-352611 A proposes a configuration using a light direction conversion element for directing the traveling direction of light rays emitted from a light source to the normal direction to a backlight exit surface, as a configuration allowing the surface of the backlight to have uniform brightness.
When configuring a surface light source to be used in a backlight of a liquid-crystal display apparatus, it is preferable to employ a lighting device that emits white light. For example, JP 2008-545269 T discloses a lighting device as shown in FIG. 12. In this lighting device, a blue LED that emits blue light is used as the LED, and a phosphor layer 102 is disposed between a blue LED 101 and a lens 103. The phosphor layer 102 converts a part of the blue light into yellow light, so that white light is generated using the blue light.
In the lighting device as shown in FIG. 12, the phosphor layer 102 has larger surface dimensions than those of the blue LED 101. In such a configuration, the blue light emitted from the blue LED 101 and the yellow light exiting the phosphor layer 102 have different light paths to reach a light exit surface of the lens. This makes the illuminance distribution of the yellow light different from that of the blue light in an area to be irradiated with the lights, resulting in unevenness of color.
Moreover, the configuration disclosed in JP 2002-352611 A is a configuration for making the brightness uniform and is not effective in reducing the unevenness of color.